The present invention is directed to the processing of digitized true color information wherein each pixel to be displayed is represented by three color planes (red, blue and green) with each plane having 8 bits for 24 bits systems or 5 bits for 16 bit systems. Referring to FIG. 1a, the video signal created by a camera or a VCR 11, which is a moving video image at e.g., 30 frames per second for NTSC or 25 frames per second for PAL is digitized, using a frame grabber 13. The digitized true color images may then be compressed 15 according to a compression algorithm, e.g., the MPEG standard for moving images, into a compressed file reduced up to 200 to 1 by MPEG and stored as a file 17 for later processing. If the captured moving video image is compressed, it must first be decompressed 19 for further processing. In the prior art, the image is passed by interface 21 to a 16 bit or 24 bit display board 24 directly connected to interface 21 which processes the signal and displays it on an RGB monitor 26. The specifics of interface 21 depend upon the 16 or 24 bit display board utilized, the details of which are well known to persons skilled in the field of the invention. It is possible to display true color moving images using an interface 21 and display board 24. However, 16 or 24 bit display boards and RGB monitors are expensive and require relatively high speed data transfers.
A still digital image is formed by a large number of pixel lines forming a matrix of rows and columns where each element is a pixel. A sequence of moving pictures is a large number of still digital images that are displayed in very rapid succession, typically 25 or 30 frames per second.
The pixel value is a number that defines the color of the pixel, according to a dictionary known to the graphics display board.
The number of bits allocated to each of these values (the picture depth) directly determines the quality of the image. In other words, the more bits used to represent the pixel value, the larger the number of colors output simultaneously to the screen.
For example, if the depth of an image is a single bit, each pixel can have one of two values only --0 and 1. If, for instance, black is defined to be 0 and white is defined to be 1, as is the case for facsimile methodologies, only bi-level images can be produced.
Systems based on gray scale images, where the image depth is 8 bits, can display 256 levels of gray simultaneously, with 0 representing black and 255 (FF.sub.16) representing white.
In systems based on true-color images, pictures consist of three spaces --R (red), G (green), B (blue) --where the depth of each is 8 bits (that is, each pixel is characterized by three 8-bit values), with 255 (FF.sub.16) representing red, green or blue (or white if each of R, B and G is 255) and 0 representing no color (i.e., black if each of R, B and G is 0). The number of colors that can be simultaneously displayed in a 24 bit true color picture is 2.sup.24 --some 16 million colors. In a 16 bit true color system wherein each of the three spaces R, B and G is represented by 5 bits, the total number of colors that can be displayed simultaneously is 2.sup.15 or some 33,000 colors.
Color images can also be displayed by another method known as the pseudo-color method. This method displays, for each pixel, a color from a reduced table or palette. Thus, instead of picking one from among approximately 16 million possible colors (24 bit systems) or 33,000 possible colors (16 bit systems), the pseudo color method allows choosing 256 colors from a much larger set of colors. However, the display (of an image, of a number of images or a moving picture sequence) is limited to 256 colors only. However, pseudo-color systems are much less expensive than true color systems. Additionally, with a pseudo color system, instead of requiring a high speed bus to transfer the data to a display device, pseudo color data may be sent over a standard AT bus having a bandwidth under 3 MBytes per second.
The prior art solution for converting a true color image to a pseudo color image is performing a statistical process on the image--called a histogram--that determines the 256 colors which most frequently appear in an image. Any other color in the picture is represented by one of these 256 possibilities. The histogram process determines, for each image, a table containing the 256 most frequently used colors.
The histogram method does not allow displaying more than one image at a time without affecting the image's optimal color table. This method is not capable of outputting a video sequence which displays a moving image of, for example, 25 or 30 images per second, each image having its own color, with currently available hardware since the histogram process requires switching color tables in the graphics board before display, a procedure that cannot be carried out fast enough to match the image succession (i.e., 25 or 30 times per second).